The objectives are to quantify and to examine the interactions of receptors on the human red cell membrane with blood group antibodies, plant and animal lectins and drugs. The methods employed include radioiodinated and enzyme-labeled antibodies combined with quantitative measurements of hemagglutination in the AutoAnalyzer continuous flow system. Endocytosis induced by various means including the action of 8- aminoquinolines will be used for topographic studies of the red cell membrane after reacting with antibodies, enzymes and chemicals in vitro and in diseases associated with abnormalities of red cell function. Supplementing these studies the solubilized proteins of red cell ghosts fractionated by gel filtration will be studied by acrylamide gel electrophoresis and by immunochemical methods using radioiodinated and enzyme-labeled antibodies and the ability to inhibit agglutination of red cells by specific blood group antibodies in the AutoAnalyzer system.